Molded case circuit breakers rated for residential and lower current industrial applications utilize both thermally responsive as well as magnetically responsive trip units for overcurrent and short circuit interruption. The thermal element responds to moderate overcurrent conditions, whereas the magnetic trip unit responds to severe overcurrent conditions. It is a current practice of the circuit protector industry to mount the magnet portion of the magnetic trip unit around the bimetal trip unit and to arrange the armature as part of the circuit breaker latching system. The magnet portion is generally U-shaped in configuration and surrounds the circuit breaker load strap or the thermal element and responds in a manner similar to that of a conventional U-shaped "slot motor" wherein the magnetic forces induced within the magnet are concentrated at the ends of the magnetic arms. The flat plate armature is arranged perpendicularly across the arms with an air gap between the arms and the armature to set the magnetic force. The magnetic force of attraction between the armature and the magnet is such that the minimum magnetic force occurs at the instant of short circuit overload since the magnet and the armature are at their furthest distance apart and the high reluctance air gap at this time is a maximum. As the armature moves toward the magnet under the magnetic force of attraction, the gap distance decreases causing the effective magnetic force to rapidly accelerate. The magnetic force is at a maximum upon the instant of contact between the armature and the magnet, at which time the air gap is effectively zero. One example of a state of the art thermal magnetic trip unit is found within U.S. Pat. No. 4,609,898 in the names of Raymond Seymour et al.
It has since been determined that the magnetic trip sensitivity can be substantially increased by utilizing a pair of opposing magnets wherein one of the magnets is fixed and the other is allowed to move, similar to an armature but opposite in direction. The magnetic forces between the magnets are of repulsion and hence the maximum magnetic force occurs at the instant of an overcurrent condition and decreases thereafter as the movable magnet becomes displaced and the high reluctance air gap correspondingly increases. In most circuit breaker designs, it is important to generate a trip force at the instant of overcurrent occurrence such that the largest magnetic trip force occurs instantaneously. In some breaker applications, the thermal trip unit is eliminated such that both long time overcurrent as well as short circuit trip functions are provided by means of a magnetic dashpot trip unit alone. A "dashpot" is an arrangement wherein a magnetic plunger is arranged within a viscous liquid which is encapsulated within a sealed container. A solenoid winding around the container generates a magnetic force in proportion to circuit current. The viscosity of the liquid provides the time overcurrent delay similar to the thermal trip unit. Upon the occurrence of a short circuit condition, the magnetic force generated by the solenoid winding is sufficient to rapidly overcome the liquid velocity to trip the breaker.
The purpose of this invention is to provide a simple magnetic trip unit having sensitive response to both overcurrent as well as short circuit conditions.